Particle Image Velocimetry – Particle Density and Neutral Buoyancy

Density of particle vs. density of fluid:

Silver Metal Coated Hollow Glass Microspheres

Many flow visualizations are done in water but there are a variety of other liquids and even water based solutions that will have differing densities whose velocity fields are being mapped or will be in the future. Therefore, having seed particles to match these differing densities is important to provide neutral buoyancy, one of the most important factors regarding flow conformity.

With polyethylene spheres contributing a range of densities that provide neutrally buoyant tracers for most water fluid flows thought needs to be directed towards furnishing tracers to match other densities. Gasoline and similar carbon based fluids are an example of areas of fluid flow mechanics where lower density tracer particles are necessary. Silver Coated Hollow Glass Microspheres have the density range to potentially work for many carbon chain liquids, with current densities ranging from .15 g/cc to .75 g/cc and possibly higher. This leaves some room for improvement in the densities between .75 g/cc and .96 g/cc.

There are many options available for neutrally buoyant microspheres in water. While, other fluids may have more difficulty finding an ideal density tracer, there are options available and scientists are working to provide more unique and targeted tracer particles for use with low density fluids.

BioCompatability of Metal Coated Spheres

For those scientists who are looking to use silver coated materials such as silver coated microspheres in biomedical applications, it is important to understand whether they are bio-compatable. A selection of abstracts and article references related to the biocompatability of silver follow:

The experiments reported have referred to some of the characteristics of the biocompatibility of Ag. Silver has been shown to display interactions with albumin, as an example of a plasma protein, quite different from those of most metals. Such studies shed further light on the complex issue of protein adsorption on biomaterials. It has also been demonstrated that Ag at concentrations < 1 ppm exerts a considerable influence on the activity of lactate dehydrogenase, this effect being reversed in the presence of albumin. A significant but transient increase in blood levels of Ag following intramuscular implantation of the metal has been observed. This is not reflected in any raised urine level. It is proposed that the richly vascular tissue immediately surrounding the implant in the acute phase of the response gives rise to the transient increase, but a subsequent decrease in vascularity reduces this possibility. It appears that Ag released from implants following this initial period substantially remains in the local area.2

Lack of toxicologocial side-effects in silver-coated megaprostheses in humans1

Deep infection of megaprostheses remains a serious complication in orthopedic tumor surgery. Furthermore, reinfection gets a raising problem in revision surgery of patients suffering from infections associated with primary endoprosthetic replacement of the knee and hip joint. These patients will need many revision surgeries and in some cases even an amputation is inevitable. Silver-coated medical devices proved their effectiveness on reducing infections, but toxic side-effects concerning some silver applications have been described as well. Our study reports about a silver-coated megaprosthesis for the first time and can exclude side-effects of silver-coated orthopedic implants in humans. The silver-levels in the blood did not exceed 56.4 parts per billion (ppb) and can be considered as non-toxic. Additionally we could exclude significant changes in liver and kidney functions measured by laboratory values. Histopathologic examination of the periprosthetic environment in two patients showed no signs of foreign body granulomas or chronic inflammation, despite distant effective silver concentrations up to 1626 ppb directly related to the prosthetic surface. In conclusion the silver-coated megaprosthesis allowed a release of silver without showing any local or systemic side-effects.1

Specific Article References for the biocompatability of silver are below: See the References

Patent Review: Process for forming hollow glass spheres up to 5mm

Hollow glass microspheres are currently commercially available in sizes of up to 0.2mm, but not larger. William Mathews et. al offer a potential solution for producing larger hollow glass microspheres in patent 3,838,998. They present a method which would enable the production of hollow glass spheres in sizes up to 5mm in diameter. As can be seen in the details of the patent, the process is quite complex, and seems to only offer pilot production capability, which would reason why we do not currently see hollow glass spheres in the 0.2-5mm range.

A process for forming hollow glass micro-spheres with walls of controllably variable thickness in a size range of 50 to 5,000 microns, embodying (1) preparation of a water slurry of finely particulated, high temperature and low temperature glass formers; (2) prilling the slurry in a vertical spray drying tower; (3) separating and supporting the individual prilled feed material; (4) heating the feed material to glassification of the high temperature glass former while maintaining appropriate geometry and shell thickness and (5) cooling the finished product. The high temperature glass former is preferably a naturally occurring soda feldspar. The process is particularly adapted to form thicker walled micro-spheres of larger size and high quality.

Metal Coated Microspheres – Conductive Silver Coating

From early days engineers have been looking for ways to shield circuits from electromagnetic interference (EMI). One of the most effective methods of shielding is by creating an electrically conductive enclosure around the circuit or device. This can be accomplished by using any electrically conductive material. Advances in coated microspheres have enabled the creation of light weight electrically conductive coatings that provide excellent EMI shielding.

Electrically conductive microspheres are produced by applying a metallic silver coating to the surface of the microspheres, thus giving the advantages of a metal particle with the additional properties of the core microsphere. Typically hollow glass microspheres are silver coated as this offers the combination of a low density filler and a conductive particle. Coatings with EMI shielding of greater than 45db have been produced by adding as little as 20% by weight of M-18 silver coated microspheres.

What is a Microsphere?

Microsphere is a term used for small spherical particles, with diameters in the micrometer range (typically 1μm to 1000μm (1mm)). Microspheres are sometimes referred to as microparticles.

Microspheres can be manufactured from various natural and synthetic materials. Glass microspheres, polymer microspheres and ceramic microspheres are commercially available. Solid and hollow microspheres vary a lot in density and, therefore, are used for different applications. Hollow microspheres are typically used as additives to lower the density of a material. Solid microspheres have numerous applications depending on what material they are constructed of and what size they are. Continue reading “What is a Microsphere?” »